Exhaust brake quit working

If it's a PAC brake, a likely cause is lack of lubrication on the pivot. It's not difficult to remove the unit, and if it can be freed, great. I had to have the exhaust brake replaced on my motorhome shortly after I got it, and it wasn't hugely expensive. I subsequently replaced it again with an upgraded brake that is designed to be effective at lower engine rpm. Again, not a huge expense.